Hexavalent chromium compounds have been used as traditional conversion coatings to treat metal surfaces to improve their corrosion resistance and paint adhesion. Hexavalent chromium shows toxicological effects and has been determined by the Environmental Protection Agency as a risk to the environment and by the Occupational Safety and Health Agency as a health risk. Moreover, chemistries based on hexavalent chromium are classified as carcinogens by these agencies.
Within the past few decades, various compositions and processes, not relying on hexavalent chromium, have been described and used for treating metal surfaces. One such example is described in U.S. Pat. No. 5,859,106 to Jones et al., which describes a composition comprising a compound of a group IV-B element and a polymer system having multiple carboxylic and hydroxyl groups.
It is highly desirable to provide coatings and processes which are free of hexavalent chromium, but still capable of improving paint adhesion and corrosion resistance of metal surfaces, such as aluminum, which are comparable to conventional hexavalent chromium-based coatings. Additionally, there is a need to provide protective coatings having excellent corrosion resistance with lowered resistivities and adequate coating weights.